Y. Kuwasawa

Observation of anomalous transition in the upper critical fields of Nb/Nb0.5Zr0.5 multilayers

Abstract An anomalous upturn in the parallel upper critical field is observed in the Nb/Nb0.5Zr0.5 multilayer with the period Λ=400 A and 500 A , as predicted by Takahashi and Tachiki for multilayer composed of two materials with different electronic diffusion constants and equal transition temperature. We have found the anomalous feature in a temperature dependence of transition width ΔHc2‖ in the region where the upturn occurs. In the angular dependence of the upper critical fields, the slope dHc2/dθ near the field direction parallel to the film surface also exhibits an abrupt change in the range of upturn.

Anisotropy of critical current and anomalous upturn feature of parallel upper critical fields in the superconducting Nb/Nb0.5Zr0.5 multilayer

Abstract Measurements of the critical current J c have been made for the Nb/Nb 0.5 Zr 0.5 multilayer which showed a sharp upturn behavior in a parallel upper critical field H c2∥ ( T ), caused by the shift of the order-parameter Δ( r ) from Nb- to NbZr-layers. In the region where Δ( r ) is localized in the Nb-layers, the peak was observed in the J c − H curves for parallel field orientation. With increasing temperature, the J c -peak value drastically decreased and its position was shifting to a lower field. In the angular dependence of the critical current density, J c (θ) had a maximum when the flux lines are exactly aligned in parallel to the layer and sharply decreased with tilting of angle. The feature is caused by an intrinsic flux pinning mechanism. In the upturn region, we found the same characteristics of J c .

Transport critical current density and dimensional crossover in superconducting Nb/NbZr multilayers

Abstract In order to understand the nature of the mixed state, the critical current density J c of Nb/NbZr multilayers was measured as a function of magnetic field H and of the angle between H and the film plane at the temperatures where the superconducting order parameter is localized in the Nb layers. In the parallel fields, which are perpendicular to the current J , the J c - H curves show a peak at H = H max . The observed angular dependence of J c below and above H max can be explained by the 3D stepwise vortex model and the 2D pancake vortex model, respectively. These results indicate that a dimensional crossover in the mixed state occurs at H max . This crossover may be the cause of the field dependence of J c which is an increasing function of H below H max .

Angular dependence of glass scaling behavior in current-voltage characteristics of CVD YBa2Cu3O7-δ films

Abstract In order to investigate the effect of intrinsic pinning on the glassy state, we have measured current-voltage ( I–V ) curves of CVD YBa 2 Cu 3 O 7- δ near the glass transition temperature T g as a function of the angle θ between magnetic field and c -axis. In the scaling analysis of I–V curves based on the vortex-glass model, it is found that the glass scaling function and the critical exponents are independent of θ, and do not show a remarkable change even at θ =90° ( H ⊥ c ). Our result indicates that the periodic plane-like intrinsic pinning is not so effective near T g and vortices are pinned in random point-like disorder for all field directions.

Correlation of homoepitaxial growth of high-T c A15 Nb3Ge with characteristics of substrate surface

Metastable Nb3Ge films were prepared by coevaporation in an ultrahigh vacuum with homoepitaxial growth onto Nb-rich A15 Nb-Ge base layers having two types of surface characteristics: one was formed with a two-phase mixture of Nb-Ge and NbO, and the other had a clean Nb-Ge phase without any impurities. The homoepitaxial films grown on the former surface had more stable and reproducible higherTc features than did those made on the latter surface. Compositional analysis near and at the interface between the homoepitaxial film and the base layer was made by Auger electron spectroscopy for representative samples and the oxide formed on the base layer was investigated by x-ray photoelectron spectroscopy.

Current-voltage characteristics of YBa2Cu3O7−δ films at low magnetic fields

Abstract We have examined the scaling behavior and the functional form of the I − V characteristics of YBa 2 Cu 3 O 7− δ films at fields H from 0.001 to 6 T applied parallel to the c -axis. In all cases, the I − V curves obey the scaling law based on the second-order phase transition, and are fitted by V / I = R 0 exp[− I 0 / I ) μ ]. At high fields, the exponents ν , z , and μ , are independent of H and are consistent with the prediction of the vortex-glass (VG) model. However, below ∼ 0.05 T, these exponents become H -dependent, which is indicative of the existence of another phase at low fields.

Preferential position for nucleation of order parameter and angular dependence of upper critical fields in Nb/NbZr multilayers

Abstract The Nb/NbZr multilayer is a typical one composed of two materials with the same bulk transition temperature and different electron diffusion constants and exhibits an anomalous upturn at T+ in the temperature dependence of parallel upper critical field Hc2‖(T). For all samples which show the upturn behavior in Hc2‖(T), we have found the anomal minimum and maximum at a tilt of angle θ = θ ∗ and a perpendicular direction θ = 0, respectively. Of particular interest is the existence of a different tendency in the variation of θ ∗ and Hc2⊥-peak as a function of T below and above T+. From our experimental results, we suggest the dimensional crossover of the localization of Δ(r) for a tilt of angle. The angular dependence of Hc2(T) has been in a good agreement with the numerical calculations.

Current-voltage characteristics and layer coupling in amorphous W/Si multilayers

Abstract Current-voltage ( I - V ) characteristics have been measured on the amorphous W monolayers and W/Si multilayers in the absence of magnetic field. For the monolayers and the multilayers with thick Si layers, the I - V curves show straight lines on a log-log scale and the Kosterlitz-Thouless transition. On the other hand, for the multilayers with thin Si layers, they show negative curvatures on such a scale at low temperatures. This is attributed to the effect of interlayer coupling giving rise to the dimensional crossover of vortex configuration. Our results are discussed with the theories which take the coupling effect into consideration.

Preferential position of order parameter and angular dependence of upper critical fields in Nb/NbZr multilayers

The Nb/NbZr multilayer, which is composed of two superconductors with the same critical temperature and the different electron diffusion constant, is a typical system to exhibit the anomalous upturn inT-dependence of upper critical fieldHc2II parallcl to the film surface. For the angular dependence ofHc2, we have observed a minimum on the tilt of θ (as θ=0o forH⊥ film). This is interpreted as the dimensional crossover of the localized order parameter at (as θ=0o forH ⊥ film). This mechanism has been also confirmed in the variation ofI-V characteristics withH-direction to the film, where the samples were located at the specific configuration in the field.

Superconducting upper critical fields of V/Cr-superlattices

Abstract In V/Cr-superlattices, the superconducting transition temperature and upper critical fields are investigated for the superlattices of various Cr-layer thickness ( d Cr =20–90 A ) under a constant V-layer thickness ( d v =300 A ). These samples are prepared by UHV-deposition at 300 and 500°C onto the V-buffer layer that is grown on the sapphire substrates heated upto 500°C. The dependence of critical temperature on the Cr-layer thickness obeys the fits calculated by the Werthamer model for the films deposited at 300°C. A dimensional crossover in the parallel upper critical fields was observed in the samples deposited at 300°C and was not found in the films done at 500°C. Such a crossover is shown in the superlattices with d Cr ≤50 A and occurs when the effective superconducting layer thickness becomes comparable to the perpendicular coherence length ξ ⊥ at the crossover points.